[BUGFIX] t3lib_extjs_ExtDirectDebug::__destruct() missing

In the Developer Logs there are complaints about the missing destructor.
Fix: add it.

Change-Id: I42595a7ebbc3c941bf64bc0699ff908059c107bb
Resolves: #25278
Reviewed-on: http://review.typo3.org/1092
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Oliver Klee
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer